To meet the increasingly stringent regulations regarding the content of exhaust gases from internal combustion engines, which are being proposed and enforced in the United States and other countries, devices have been developed to purify the exhaust gases. One type of purification device is connected to the exhaust system of a vehicle, such as an automobile, so as to treat gases exhausted from the vehicle engine cylinders before the gases are released to the atmosphere. Typically, the device includes an appropriate catalyst, generally in the form of an active coating on a carrier material or member, for converting noxious exhaust gas components, such as carbon monoxide, uncombusted hydrocarbons, and nitric oxides, into harmless components.
One such device for catalytic exhaust gas purification or emission control is described and illustrated in German Auslegeschrift (DAS) No. 1,476,507 and in corresponding U.S. Pat. No. 3,441,381. The catalytic device of the German publication includes a housing and a monolithic carrier member for a catalyst. The catalyst carrier is supported in the housing by a layer of elastic material disposed between the carrier and the wall of the housing. The particular elastic support described and illustrated in the German publication is a corrugated member fabricated of wire mesh and encircling the carrier. In another catalytic device, as described in commonly owned, copending U.S. patent application Ser. No. 403,270, filed Oct. 3, 1973, the elastic support is a prestressed fibrous ceramic material, such as aluminum silicate. As noted in that application, a suitable material of this type is sold under the trademark "Fiberfrax".
In catalytic converters of the type described and illustrated in the German publication and the U.S. Patent application, a compressive stress on the elastic support must be maintained at or above a predetermined minimum value throughout the entire range of operating temperatures of the associated internal combustion engine in order to insure that the catalyst carrier will always be securely supported in the housing. In the lower performance range of an internal combustion engine, during which the temperature of the exhaust gas from the engine is comparatively low, the requirement of a minimum compressive stress is easily met. In the upper range of engine performance, however, the exhaust gas may reach a temperature high enough to have an adverse effect on the functioning of the elastic support. Specifically, except for the portion of the housing where the elastic support mounts the monolithic carrier, the exhaust gas flowing through the converter directly contacts the full surface area of the walls of the housing. Consequently, the housing walls fully exposed to the gas may be quickly heated to very high temperatures. The high temperatures of the exposed walls are then transmitted throughout the housing, because of the heat conductivity of the housing material, so that even in the portion of the housing where the carrier is mounted, the temperature of the housing is remarkably above the ambient atmospheric temperature. The heated housing expands away from the catalyst carrier and the compressive stress on the elastic support is correspondingly reduced. As the compressive stress is reduced, the elastic support holds the carrier less firmly in the housing. When subjected to resonant vibrations produced by the vehicle engine, the carrier can shake loose from the housing and be damaged or otherwise adversely affect the operation of the converter.
Since the catalyst carrier is generally located in the central longitudinal portion of a converter housing, the end portions of the housing, which are directly and fully contacted by the exhaust gas, tend to attain higher temperatures than the central portion. In addition to the overall expansion of the housing, therefore, the ends of the housing expand to a greater extent because of their higher temperatures. The differential expansion tends to reduce the length of housing wall applying compressive stress on the elastic support member. Thus, such differential expansion of the housing also facilitates the carrier shaking loose and the resultant adverse effects on the operation of the converter.